Crash in QWindowsWindow::setDropSiteEnabled

A crash occurs in QWindowsWindow::setDropSiteEnabled when the window is destroyed. CoLockObjectExternal is referencing m_dropTarget after its refcount has already been dropped to zero and the object has been released. The crash message is an unhandled exception in ole32.dll, access violation reading location 0xFFFFFFFFFFFFFFFF.

I have confirmed that the crash is resolved by first unlocking the object and then releasing it, and I have attached the appropriate patch.

I have not attempted to find the minimal application code required to reproduce the issue, but in our application it occurs when clicking the close button on the window. I discovered the issue using Qt 5.4.1, however from inspecting the code in 5.5.0, I see the same problem is present.

Interestingly, the very same problem of calling CoLockObjectExternal in the wrong order was discovered and fixed several years ago in a similar bit of code, see QTBUG-13237.